Influences of controlled microwave field irradiation on physicochemical property and methane adsorption and desorption capability of coals: Implications for coalbed methane (CBM) production

Abstract The influences of microwave irradiation on physicochemical properties and methane adsorption and desorption capability of four ranks of coals were investigated to explore the feasibility of enhancing coalbed methane (CBM) production by using controlled microwave field irradiation. The results indicate that the microwave irradiation changes coal pore structure characteristics. In particular, the micropore surface area and volume of low-rank coals decrease, whereas those of medium- and high-rank coals increase slightly. The mesopore surface area and volume of all the coals after microwave irradiation decrease. Overall, microwave irradiation decreases the adsorption pore volume with diameter less than 7.00 nm of all the coals for CH4. Furthermore, microwave irradiation enhances the connectivity of adsorption pores and seepage pores of low-rank coals and decreases the connectivity of adsorption pores of medium- and high-rank coals. With respect to surface chemistry property, microwave irradiation reduces the total amount of primary oxygenic groups including C–O, C = O and –COOH of all the coals. Finally, microwave irradiation decreases the maximum CH4 adsorption capability on all the coals by 2.50–18.18%, because of the decreasing number of adsorption pores for CH4. Moreover, microwave irradiation weakens CH4 adsorption and desorption hysteresis on coals, probably due to the decreasing CH4 adsorption-induced coal matrix swelling. The decreasing hysteresis implies that controlled microwave field irradiation is beneficial to enhancing CBM recovery.